From 58300f4743231724686d9ddf481aeefa4f90d2f7 Mon Sep 17 00:00:00 2001
From: Derrick Stolee <dstolee@microsoft.com>
Date: Tue, 30 Mar 2021 13:10:59 +0000
Subject: [PATCH] sparse-index: add index.sparse config option

When enabled, this config option signals that index writes should
attempt to use sparse-directory entries.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 Documentation/config/index.txt |  5 +++++
 cache.h                        |  1 +
 repo-settings.c                |  7 +++++++
 repository.h                   |  3 ++-
 sparse-index.c                 | 34 +++++++++++++++++++++++++++++-----
 5 files changed, 44 insertions(+), 6 deletions(-)

diff --git a/Documentation/config/index.txt b/Documentation/config/index.txt
index 7cb50b37e9..75f3a2d105 100644
--- a/Documentation/config/index.txt
+++ b/Documentation/config/index.txt
@@ -14,6 +14,11 @@ index.recordOffsetTable::
 	Defaults to 'true' if index.threads has been explicitly enabled,
 	'false' otherwise.
 
+index.sparse::
+	When enabled, write the index using sparse-directory entries. This
+	has no effect unless `core.sparseCheckout` and
+	`core.sparseCheckoutCone` are both enabled. Defaults to 'false'.
+
 index.threads::
 	Specifies the number of threads to spawn when loading the index.
 	This is meant to reduce index load time on multiprocessor machines.
diff --git a/cache.h b/cache.h
index 74b43aaa2b..8aede373ae 100644
--- a/cache.h
+++ b/cache.h
@@ -1059,6 +1059,7 @@ struct repository_format {
 	int worktree_config;
 	int is_bare;
 	int hash_algo;
+	int sparse_index;
 	char *work_tree;
 	struct string_list unknown_extensions;
 	struct string_list v1_only_extensions;
diff --git a/repo-settings.c b/repo-settings.c
index d63569e404..0cfe8b787d 100644
--- a/repo-settings.c
+++ b/repo-settings.c
@@ -85,4 +85,11 @@ void prepare_repo_settings(struct repository *r)
 	 * removed.
 	 */
 	r->settings.command_requires_full_index = 1;
+
+	/*
+	 * Initialize this as off.
+	 */
+	r->settings.sparse_index = 0;
+	if (!repo_config_get_bool(r, "index.sparse", &value) && value)
+		r->settings.sparse_index = 1;
 }
diff --git a/repository.h b/repository.h
index e06a230156..a45f7520fd 100644
--- a/repository.h
+++ b/repository.h
@@ -42,7 +42,8 @@ struct repo_settings {
 
 	int core_multi_pack_index;
 
-	unsigned command_requires_full_index:1;
+	unsigned command_requires_full_index:1,
+		 sparse_index:1;
 };
 
 struct repository {
diff --git a/sparse-index.c b/sparse-index.c
index 7631f7bd00..6f4d95d35b 100644
--- a/sparse-index.c
+++ b/sparse-index.c
@@ -102,19 +102,43 @@ static int convert_to_sparse_rec(struct index_state *istate,
 	return num_converted - start_converted;
 }
 
+static int enable_sparse_index(struct repository *repo)
+{
+	const char *config_path = repo_git_path(repo, "config.worktree");
+
+	git_config_set_in_file_gently(config_path,
+				      "index.sparse",
+				      "true");
+
+	prepare_repo_settings(repo);
+	repo->settings.sparse_index = 1;
+	return 0;
+}
+
 int convert_to_sparse(struct index_state *istate)
 {
 	if (istate->split_index || istate->sparse_index ||
 	    !core_apply_sparse_checkout || !core_sparse_checkout_cone)
 		return 0;
 
+	if (!istate->repo)
+		istate->repo = the_repository;
+
 	/*
-	 * For now, only create a sparse index with the
-	 * GIT_TEST_SPARSE_INDEX environment variable. We will relax
-	 * this once we have a proper way to opt-in (and later still,
-	 * opt-out).
+	 * The GIT_TEST_SPARSE_INDEX environment variable triggers the
+	 * index.sparse config variable to be on.
 	 */
-	if (!git_env_bool("GIT_TEST_SPARSE_INDEX", 0))
+	if (git_env_bool("GIT_TEST_SPARSE_INDEX", 0)) {
+		int err = enable_sparse_index(istate->repo);
+		if (err < 0)
+			return err;
+	}
+
+	/*
+	 * Only convert to sparse if index.sparse is set.
+	 */
+	prepare_repo_settings(istate->repo);
+	if (!istate->repo->settings.sparse_index)
 		return 0;
 
 	if (!istate->sparse_checkout_patterns) {